ACVIM 2017: Pituitary Pars Intermedia Dysfunction in Horses

Pituitary pars intermedia dysfunction (PPID), also known as equine Cushing’s disease, is an endocrine disorder causing hyperadrenocorticism in horses. Two poster presentations at the 2017 ACVIM conference in National Harbor, Maryland, reported the results of recent research into equine PPID.

Effects of Dexamethasone on Plasma ACTH and Cortisol Concentrations

A research group from the University of Missouri conducted a pilot study to examine the effects of dexamethasone administration on plasma adrenocorticotropic hormone (ACTH) and cortisol concentrations in healthy horses and horses with PPID.

In horses, PPID can be diagnosed by an increase in endogenous plasma ACTH concentration. The effect of dexamethasone administration on endogenous ACTH levels in horses has not been reported, write the authors. Previous studies have indicated that dexamethasone suppresses basal cortisol levels in healthy horses but not in horses with PPID. However, the authors note that this effect is inconsistent.

The pilot study, which was not randomized or blinded, included 6 horses with PPID and 6 healthy horses, all over 11 years of age. The investigators collected blood samples from each horse immediately before a single intravenous dose of dexamethasone and every 6 hours for 48 hours afterward.

In all 12 horses, plasma ACTH and cortisol concentrations decreased significantly after dexamethasone administration, with no significant differences between the groups. Dexamethasone suppressed ACTH concentrations for about 36 hours and suppressed cortisol concentrations for about 48 hours in all horses.

The authors did not anticipate that dexamethasone would suppress ACTH and cortisol levels in horses with PPID to the extent indicated by the results. They conclude, “Routine diagnostic testing for PPID should be interpreted with caution in horses concurrently receiving exogenous corticosteroid therapy for other comorbidities.”

PPID in Horses in the Southern Hemisphere

The results of tests for equine PPID (endogenous ACTH measurement and dexamethasone suppression testing) vary according to the season of the year. Investigators from the University of Queensland, Australia, hypothesized that clinical features of PPID in horses in the Southern Hemisphere would differ from those reported in horses the Northern Hemisphere.

The researchers retrospectively examined the records of 99 horses diagnosed with PPID at the University of Queensland. The most common signs were tachypnea, anhidrosis, hirsutism, weight loss, polyuria/polydipsia, and lameness. Eighty-two percent of the horses had laminitis, 35% had insulin dysregulation, and 100% had gastrointestinal parasites.

Eighty-three percent of the horses were treated with pergolide. Clinical signs improved in 80%, and 65% survived. Younger age and absence of weight loss were associated with laminitis. Younger age, duration of clinical signs, and the need for higher pergolide doses were associated with insulin dysregulation. Clinical improvement was associated with survival.

“Commonly described features of PPID are found in the tropical and subtropical climates of the Southern Hemisphere, but more unusual clinical signs can also warrant investigation of PPID,” conclude the authors. They recommend that insulin dysregulation be considered in younger horses that do not respond to pergolide.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is also a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC. She works as a full-time freelance medical writer and editor and continues to see patients a few days each month.